A charge coupled device (CCD) image sensor or a CMOS image sensor (CIS) may be generally used as an image sensor.
A CIS may be classified as an active pixel sensor (APS) or a passive pixel sensor (PPS). APSs may be widely used due to their advantageous characteristics and operating speed. CISs may be used in low-priced digital still cameras. CCDs may be used in video cameras.
Generally, CISs may be manufactured using a CMOS process. Accordingly, CISs may have lower fabrication costs and lower power consumption compared to CCDs. For this reason, CISs may be widely used in battery-operated devices such as mobile phones and other mobile terminals.
CISs may include a photodiode as a photoelectric conversion element. CISs may measure the amount of electric charge integrated in the photodiode using a drive transistor or the like in a pixel to convert the intensity of incident light into electrical signals.
CISs may be classified into three-transistor (3T) type CISs, four-transistor (4T) type CISs, etc. A 3T type CIS may include a photodiode, a reset transistor, a source follower transistor and a select transistor. A 4T type CIS may further include a transfer transistor provided between the photodiode and the reset transistor in addition to the elements of the 3T type CIS.
FIG. 1 is an equivalent circuit diagram of a conventional unit pixel in a 4T type CIS.
Referring to FIG. 1, a conventional unit pixel 10 may include a reset transistor 12, a transfer transistor 14, a drive transistor 16, a select transistor 18, and a floating diffusion FD formed at a connection point between the transfer transistor 14 and the reset transistor 12.
An operation thereof will be briefly described. In the conventional 4T type CIS, the floating diffusion FD may be set to a reset level using the reset transistor 12. Afterward, the 4T type CIS may drive the transfer transistor 14 to transfer photocharges integrated in a photodiode 11 to the floating diffusion FD, and therefore vary a potential thereof. The potential variation may be outputted to a signal line via the drive transistor 16. A noise-removed signal may be outputted by detecting a difference between a reset potential of the floating diffusion FD and a potential of the floating diffusion FD after the charges of the photodiode 11 are transferred thereto.
The conventional 4T type image sensor may perform a photocharge transfer function of providing a transfer path so that a transfer transistor 14 may transfer photocharges to the floating diffusion column lines CL1 to CLm. The time taken to perform this function may be referred to as an effective integration time (EIT). Simultaneously, the conventional 4T type image sensor may also perform a photocharge discharge function of providing a discharge path so that the transfer transistor together with a reset transistor 12 may discharge unnecessary photocharges integrated in the photodiode 11. The time taken to perform this function may be referred to as a non-effective integration time (NIT).
The transfer transistor 14 of the conventional image sensor as illustrated in FIG. 1, may repeatedly extract charges integrated in the photodiode 11. That is, the charges may be transferred through a path along the photodiode 11, the transfer transistor 14, the floating diffusion FD, the reset transistor 12, and the power supply voltage (VDD) terminal in sequence.
When a row select signal is inputted into the select transistor 18, the drive transistor 16 may drive an output terminal (EIT section). When a row select signal is not be inputted into the select transistor 18, the drive transistor 16 may not drive the output terminal (NIT section).
Recently, there has been provided a 5T type APS in which an overflow drain transistor may be further provided to prevent an overflow of photocharges generated at the photodiode.
As the performance of image sensors increases, the number of transistors in a pixel may also increase, and therefore, a fill factor, which is a ratio of a photodiode area to a pixel area, may decrease. As a result, the photosensitivity of the image sensor may deteriorate.